Semiconductor integrated circuits generally have successive layers of semiconductor devices and metal conductors, each layer being separated by an intermediate dielectric layer. Electrical connections between semiconductors and/or conductors on different layers are accomplished by metal plugs which extend through the dielectric. A plug generally is fabricated by etching a hole which extends completely through a dielectric layer, and then depositing a metal so as to fill the hole.
Aluminum (optionally doped with copper) is a desirable material for plugs because of its high electrical conductivity. However, there are a number of difficulties in fabricating aluminum plugs.
One difficulty is completely filling each hole with aluminum so as to not leave any unfilled "void", within the hole, because voids increase the electrical resistance of the plug and create mechanical instability which can lead to broken connections. To maximize the density of an integrated circuit, the holes must be as narrow as possible and must have a high aspect ratio, that is, a high ratio of length to width. It is difficult to fill a narrow, high aspect ratio hole without voids.
One potential source of voids is poor adhesion between aluminum and silicon dioxide, the most commonly used dielectric, which can cause the aluminum plug to separate or "de-wet" from the side wall of the hole, thereby creating a void. A conventional solution to this problem is to deposit a wetting layer of titanium on the wall of the hole before filling the hole with aluminum. However, when depositing aluminum over titanium, the titanium and aluminum interdiffuse and react to form titanium trialuminide (TiAl.sub.3), which causes a number of problems. One problem caused by the formation of TiAl.sub.3 is an increase in the resistance of the aluminum plug, because TiAl.sub.3 has much higher resistivity than aluminum. (P. R. Besser et al., "Effect of Si on TiAl.sub.3 Formation in TiAl Alloy Bilayers," Mat. Res. Soc. Symp. Proc., vol. 355, pp. 631-636, 1995.) Another problem caused by the formation of TiAl.sub.3 is the creation of voids within the plug, because TiAl.sub.3 occupies less volume than the elemental titanium and aluminum consumed to produce the TiAl.sub.3. (R. K. Nahar et al., "Effect of Si on the reaction kinetics of Ti/AlSi bilayer structures,"
Appl. Phys. Lett., vol. 50, no. 3, pp. 130-131, 1987.) A third problem is that the formation of TiAl.sub.3 on the side wall of a hole narrows the aperture of the hole, and thereby impedes filling the remainder of the hole with aluminum.
One method that has been proposed for inhibiting interdiffusion between aluminum and an underlying titanium layer is to dope the aluminum with a small amount of silicon. (P. R. Besser et al., supra; and R. K. Nahar et al., supra.) However, incorporating any silicon in an aluminum plug is undesirable because conventional chemistries for etching aluminum will not react significantly with silicon, hence will leave behind unreacted silicon in the form of solid particles which can contaminate the semiconductor workpiece.